Diazonium compounds are widely used as photosensitive materials for presensitized printing materials, and the most commonly employed are diazo resins typically exemplified by a formaldehyde condensate of p-diazodiphenylamine.
Compositions containing the diazo resins for forming a photosensitive layer of presensitized printing materials are classified into the type in which the diazo resin is used alone without using any binder as described, e.g., in U.S. Pat. No. 2,714,066 and the type in which the diazo resin is mixed with a binder as described, e.g., in Japanese Patent Application (OPI) No. 30604/75 (the term "OPI" as used herein means "unexamined published Japanese patent application"). In many of the latest presensitized printing plate precursors, the photosensitive layer comprises the diazonium compound and a polymeric binder to obtain the presensitized printing plate having long press life.
The photosensitive layer containing a polymer binder includes a so-called alkali development type in which unexposed areas are removed (developed) with an aqueous alkali developer and a so-called solvent development type in which unexposed areas are removed with an organic solvent type developer, and these types depend on the properties of the binder used. From the standpoint of safety and health of the working environment, the former alkali development type has lately attracted attention. Known methods of rendering binders alkali-developable include introduction of carboxylic acids into polymers by copolymerizing a carboxylic acid-containing comonomer as disclosed in Japanese Patent Application (OPI) No. 30604/75 or by reacting a hydroxyl group of polyvinyl alcohol with a cyclic acid anhydride, e.g., phthalic anhydride, as disclosed in U.S. Pat. No. 2,861,058. However, the polymers obtained by these methods exhibit poor abrasion resistance arising from their structure, and presensitized lithographic printing plate precursors containing such binders in the photosensitive layer only provide lithographic printing plates of low printing durability, i.e., short press life. On the other hand, polyvinyl acetal is disadvantageous in that it provides presensitized printing plate precursors of organic solvent development type only, though forming a tough and abrasion resistant film.
In addition, polyurethane resins are known to exhibit excellent abrasion resistance. Examples of application of polyurethane resins include a combination of a diazonium compound and a substantially linear polyurethane resin and a combination of a diazonium salt polycondensate and a branched polyurethane resin as described in U.S. Pat. No. 3,660,097 corresponding to Japanese Patent Publication No. 36961/74 and U.S. Pat. No. 4,337,307 corresponding to Japanese Patent Application (OPI) No. 94346/81. However, none of these polyurethane resins has an alkali-soluble group so that solubility in an aqueous alkali developer is insufficient in its nature. It has been, therefore, very difficult to carry out development without leaving some film undeveloped.